The Rochow reaction is typically employed in the industrial process for the synthesis of organohalosilanes such as methylchlorosilanes. The Rochow reaction is the direct reaction of organic halides such as alkyl halides and phenyl halides with metallic silicon particles which is carried out at 250 to 500.degree. C. in the presence of a copper catalyst and a co-catalyst. While this reaction requires keeping a high reaction rate, a key technology in the synthesis of methylchlorosilanes is to increase the selectivity of the most desirable dimethyldichlorosilane. A key technology in the synthesis of phenylsilanes is to produce the desirable diphenyldichlorosilane and phenyltrichlorosilane in a composition matching with their demand.
The conventional Rochow reaction requires a very long time for activation until the reaction reaches a steady state. The steady state, in turn, is relatively short. The contact mass's activity lowers with the lapse of time, and the yield of diorganodichlorosilane decreases accordingly. In the synthesis of methylsilanes, for example, there arise problems that high-boiling fractions such as disilanes and undesired products such as methyltrichlorosilane increase due to side reaction. This necessitates exchanging the contact mass in the reactor. Shortening the activation time is one of the outstanding problems. Since the Rochow reaction mainly uses reaction in a fluidized bed or agitating fluidized bed, a variety of reports have been made on the particle size of metallic silicon particles suitable to form the fluidized bed.
In this reaction, it is important to increase the reaction rate of metallic silicon because the cost of metallic silicon is predominant among the raw material cost. Since a variety of by-products usually form in addition to the desired diorganodichlorosilane, it is also important to control reaction conditions so that the proportion of these by-products may comply with the supply/demand balance of organochlorosilanes. Industrially, this reaction is generally carried out in a reactor such as a fluidized bed, vibrating fluidized bed or agitating fluidized bed while replenishing the contact mass to the reaction system. The reaction is a very complex gas-solid heterogeneous reaction in that the reaction itself occurs on surfaces of metallic silicon particles and the catalyst is solid. For this reason, the reaction mechanism has not been well understood. It is empirically known that the results of reaction vary over a wide range depending on the attributes (including source, manufacturer, manufacturing equipment, and crushing technique) of particular metallic silicon particles used. Several proposals have been made in this regard, but none of them have become established. In the present status, when metallic silicon of a new lot or origin becomes available, a preliminary reaction test must be done to determine whether or not it can be used in practice. Since many factors of metallic silicon that affect the reaction have not yet been revealed, lively discussions have recently been made in the society of metallic silicon (see, for example, Silicon for the Chemical Industry IV: Geirenger, Norway, Jun. 3-5, 1998).
What is important is the reaction activity of metallic silicon particles subject to reaction. The reaction activity has been studied from various aspects. In this regard, a variety of proposals (for example, relating to properties of metallic silicon itself) have been made. More specifically, it is well known that aluminum which is present in metallic silicon as an impurity is effective as a co-catalyst for the Rochow reaction. Aluminum at the same level is active in some form, but inactive in other form. For the reason that only the active form of aluminum present in metallic silicon as an impurity is necessary, H. M. Rong et al. reported the method of measuring active aluminum and recommended the use of active aluminum (see Proceeding Silicon for the Chemical Industry, pp. 69 (1998)). U.S. Pat. No. 5,334,738 or JP-A 6-234776 discloses a method for quantitatively determining the dispersion of intermetallic compounds in metallic silicon as an impurity and the criterion of choice of metallic silicon for reactivity control. This method involves cutting a metallic silicon mass, polishing the surface to a mirror finish, observing the morphology of the surface under a microscope, and computing a structural parameter QF from structural factors. Metallic silicon having the structural parameter QF of 18 to 60 has the highest reactivity and its use is recommended. U.S. Pat. No. 5,281,739 discloses to evaluate the reactivity of a contact mass by adding copper to molten metallic silicon.
Making follow-up tests on these methods, we found that the reactivity of metallic silicon could not be determined by any of these methods while these methods were effective only in special limited systems. These methods cannot be universally adopted.
In general, metallic silicon remains stable in that it has been oxidized on its surface and is covered with stable silicon oxide so that inward oxidation may not proceed beyond a certain thickness. However, as seen from the semiconductor silicon, it is well known that silicon itself has a very high oxidizing ability and there is not available metallic silicon which is free of oxide film in air. It is also known that the surface of metallic silicon particles for use in the Rochow reaction has more or less oxide film, which affects the Rochow reaction. The oxide film on metallic silicon relative to reactivity and selectivity in methylsilane reaction is discussed in the reports of G. J. Hutching et al., Silicon for the Chemical Industry, Geirenger, Norway, pp. 85-98, 1992, and G. Laroze, Silicon for the Chemical Industry II, Leon, Norway, pp. 121-127, 1994. These reports describe the influence of oxide film on reactivity and selectivity while the oxide film on metallic silicon particles is locally analyzed by x-ray photoelectron spectroscopy. Also, J. L. Falconer et al., J. Catal., vol. 159, pp. 31-41, 1996, study an oxide film on a silicon wafer and discuss the orientation of crystals and reactivity. This discussion is not applicable to the surface of metallic silicon particles for the Rochow reaction. None of the foregoing reports establish a measurement method for specifying metallic silicon particles for industrial use.
As understood from the above, the heretofore proposed methods are not generally applicable to the industrial use. When the Rochow reaction was actually carried out using metallic silicon particles which were selected under any of the foregoing selection criteria while the remaining factors are set identical, the results of reaction experienced a wide range of variation. Therefore, metallic silicon particles having properties specified by any of the prior art proposals are applicable to only a special reaction system. There is a need to have metallic silicon particles capable of finding practical use in the industry and their evaluation method.